In the United States, the Federal Communication Commission (FCC) sets the rules that govern access to wireless bandwidth or spectrum. These rules have lead to reservation of spectrum chunks for specific purposes. For example, the 824-849 MHz and 1.85-1.99 GHz frequency bands are reserved for licensed cellular and PCS services and require a valid FCC license, whereas the 902-928 MHz, 2.40-2.50 GHz, 5.15-5.35 GHz and 5.725-5.825 GHz frequency ranges are reserved as free-for-all unlicensed bands. This strict, long-term spectrum allocation is space and time invariant and any changes to it happen under strict FCC control.
Such static partitioning of spectrum bands has led to several problems brought to light by recent spectrum utilization measurements. More specifically, certain frequency bands, such as the cellular and PCS bands, are quite well utilized, however, a large part of allocated spectrum is highly underutilized. The utilization in several licensed bands varies dramatically over time and space. On the other hand, unlicensed bands (such as ISM, U-NII) have experienced significant interference due to uncoordinated, aggressive deployment, leading to overcrowding and poor network guarantees. Current FCC rules have left very little spectrum for allocation to new services or for the expansion of existing services, leading to an artificial spectrum scarcity, even though a large portion of usable spectrum remains underutilized. In other words, current spectrum usage is access limited rather than throughput limited.
Often times, during the peak hour operations, end users of a cellular network experience poor service due to peak usage on carrier frequencies provisioned by the service provider. During such peak loads, if the service provider can dynamically add capacity without having to statically acquire and configure the extra spectrum, it can incur significant cost savings and offer better quality-of-service to end users.
Consider a region, where a service provider X offers voice and data services using a license in cellular (A, B) or PCS (A-F) bands and a provider Y operates emergency response network in public safety bands. Often, at any given time and location, when provider X is experiencing high demands, provider Y may experience underutilization. If provider X can dynamically add spectrum from provider Y to its network to alleviate congestion events and then return the spectrum, it can result in high spectrum utilization and potentially less amount of spectrum for a given aggregate demand of two providers. Such lack of significant spatial and temporal correlation may exist to some extent even among providers operating the same types of services (voice, data) in the cellular/PCS band. Similarly, if the unlicensed bands can be expanded dynamically in time and space to increase available spectrum, network throughput and guarantees can be improved.
Recent technology trends and early policy trends indicate the feasibility of such opportunistic, statistically multiplexed, adaptive access to spectrum; often termed as the new paradigm of Dynamic Spectrum Access. These trends include, for example, Software Defined Radio (SDR), which takes advantage of advances in smart antennas, high bandwidth ND conversion, low power amplifiers, fast digital signal processors and inexpensive reconfigurable field programmable gate arrays (FPGAs). SDRs enable on-the-fly changes to characteristics of radio such as power, modulation, waveform, and MAC and allow same hardware to be reconfigured for use in different parts of the radio spectrum. Hardware capable of tuning to any part of a large range of frequency spectrum (i.e., 5 MHz to 6 GHz) has also been demonstrated. Such spectrum sensing enables real-time measurements of spectrum occupancy and inference on underutilized portions of the spectrum. Spectrum sensing combined with SDR and policy specific functions enable the attainment of desirable frontier known as Adaptive Cognitive Radio (ACR) that adapts based on its awareness of locale and spectrum. Such a technique allows spectrum to be managed and utilized based on real-time sensing and decision-making. The regulating bodies in the USA and European Union are slowly taking measures to alter existing policy to allow networks where spectrum is dynamically managed.